Question

Each time charles, who has a lisp, says "mithithippi" or the like, his wife, evelyn, yells, "idiot!" however, there is no evidence that evelyn's efforts to reduce the frequency of such mispronunciations have been effective. we can therefore conclude that

Answer 1

We can conclude that Charles' behavior hasn't been punished. 
His wife is verbally abusing him, but that is not strong enough an incentive for him to try and stop lisping. He never gave it a shot trying to fix this problem, and his wife (and his parents before her) didn't punish him enough for him to want to fix it. Therefore it is unlikely he will ever find a solution to the problem.